You can manage all your contacts for a client in the Contacts tab of this client.
You can easily add a new contact (using the Add button) or edit the existing ones (using the pencil button). That's also in this tab that you can define if these contacts should automatically receive the invoices and quotes you send using LSP.expert. You simply have to tick the corresponding box after clicking on the pencil. The little envelope is green when they should receive these documents, and red when they shouldn't.
Then, once you're in the document itself, you can easily send this document to the contacts concerned by clicking on Actions > Email invoice/quote.
You'll arrive in the sending screen, where you see that the contacts concerned are ticked (depending on the type of document and on what you defined at the client level). You can of course untick them and add a new contact (with an automatic update at the client level).